LEACHING AND DISSOLVING 345 has gone into solution we have quite a different state. The film above referred to not being continually reformed disappears and the entire solution becomes homogeneous within the pores of the solid as well as without. The above applies more particularly to agitation for dissolution where the soluble component is a large constituent of the material to be leached. In many cases as in the treatment of gold ores by the cyanide process it often happens that the volume of the metal to be recovered is an infinitesimal portion of the whole. For instance, in recovering $5 per ton or H-oz. of gold its volume would be about one-millionth and this would occur largely in minute fissures so that the time required for diffusion through the pores of the insoluble material will be the limiting feature and a violent agitation will be useless. Temperature.—The general effect of increased temperature is to increase the velocity of dissolution irrespective of the increase when the substance is more soluble in this hot solvent than the cold. This increase is small and does not warrant heating solvents. For instance salt in water has a solubility curve which varies little with temperature and though the velocity of dissolution in hot water would be slightly greater than in cold, the difference is not found to be sufficient to increase the fuel consumption to heat the solvent (see table of Solubilities, Appendix). Selective Dissolutions.—By selective dissolution is meant the process of dissolving one compound to the exclusion of another from a solid made up of two or more compounds classed as soluble. The following means are resorted to in this work. 1. Time of contact of solution with solids. 2. Temperature. 3. Solvent. 4. Use of permeable membranes. In applying these it is generally a combination of one or more of these methods that is used. In leaching tan bark for instance it is found that the first two, i.e., time and temperature, affect the leaching. The tan bark contains in addition to the tannin which we wish to leach from the bark some coloring matter which is soluble and would color the leather if it got into the extracts. The coloring matter however is only very slowly soluble in water of about 170°F. though it is easily soluble in water of higher temperatures. It is necessary, however, to heat the solvent (water) to about 170°F. to extract the tannin. Experience has shown with proper temperatures that when the liquors begin to color up the tannin content should be about right, due to their relative speexls of dissolution. If the solution is too hot however, they will begin to color up before the tannin is up to strength as the velocity of dissolution of the coloring matter increases with temperature more rapidly than does the velocity of dissolution of the tannin. Solubility tables arc naturally utilized for regulating selective dissolutions but in using thorn the effects of other soluble compounds on the solubility of the compound in question must be determined as its effect may be quite marked. For instance in Fig. 4, Curve 1 shows the solubility of sodium sulphate in pure water while Curve 2 shows its solubility in a solution of salt.1 The third method (that is, the use of suitable selective solvents) is almost universally used in effecting selective dissolution. It may be used in connection with the temperature effect as in leaching caliche (nitre ores) or alone as in organic work. JThere are some published tables in Comey, Seidell and the "Tables Annuelles" of the International Union of Pure and Applied Chemistry giving solubilities of two and three salts simultaneously but in general experimentation is necessary in any problem involving complex solutions.